Depending on the state of the air on entering a spray washer, it can be humidified or dehumidified. Humidification in the presence of moisture is understandable, but dehumidification is less easy to comprehend. It occurs when the spray is at a lower temperature than the air and the dew point of the air. In this condition the vapour pressure of the spray will be less than that of moisture in the air and some moisture from the air will transfer into the spray water. Hence, dehumidification.

Washers also remove some of the suspended dirt. Spray water pressure is usually between 200 and 300kPa. Air velocity through the washer is between 2 and 2·5 m/s. Spray washers must be cleaned periodically and treated to neutralise any bacteria which could be living in the water. Water quality must also be monitored and findings documented. With numerous outbreaks of Legionnaires ‘disease originating from air- conditioning systems, the Health and Safety Executive have identified these spray washers as a possible health risk.

Contemporary air-processing units may incorporate steam injection humidifiers, but unlike washers, these should not be located immediately after the cooler coil. Here, the air will be close to saturation or even saturated (100% RH) and unable to accept further moisture. Therefore dry saturated steam at over 200°C is better injected into the air close to its final discharge.

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